Aluminum alloy sheet for forming and automobile member

ABSTRACT

Provided is a 5xxx aluminum alloy sheet for forming, which sheet has low elongation anisotropy (in-plane anisotropy) and excellent press formability while having strength maintained at high level. This aluminum alloy sheet is an Al—Mg alloy sheet having a specific chemical composition. In the texture of the sheet, the orientation densities of Brass orientation and S orientation relative to a random orientation are controlled and minimized, where the orientation densities are expressed by orientation distribution functions. This allows the aluminum alloy sheet to have lower elongation anisotropy to thereby have better press formability.

FIELD OF INVENTION

The present invention relates to a 5xxx aluminum alloy sheet (rolledsheet) for forming, which has satisfactorily low elongation anisotropy.

BACKGROUND OF INVENTION

Demands have been made to provide better fuel efficiency oftransportation equipment such as automobiles, so as to cope with globalenvironmental issues caused by emission gas from such equipment. To meetthese demands, aluminum alloy materials are more and more applied, inparticular, to automobile members, instead of conventionally-usedsteels, because the aluminum alloys have lighter weights as comparedwith the steels. Representative, but non-limiting examples of automobilemembers made from aluminum alloy sheets include thin panels (bodypanels) including outer panels (exterior trim parts) such as hoods,fenders, doors, and roofs; and inner panels (interior parts), which aremade from (formed from) material sheets by press forming.

However, such aluminum alloy sheets, although having strength at thesame level with steel sheets, have inferior press formability to thesteel sheets. Thus, strong demands have been made to allow the aluminumsheets to have better press formability.

Among aluminum alloys, 5xxx aluminum alloys such as Al—Mg JIS 5052 alloyand JIS 5182 alloy have been used as aluminum alloys having relativelyexcellent press formability. In addition, various techniques have beenproposed to allow these 5xxx aluminum alloys to have better formabilityby metallurgical means such as adjustments of chemical composition,average grain size, and/or texture.

Among these metallurgical means, various proposals have been made on thetexture. For example, Japanese Patent No. 4326009, which is a divisionalapplication of Japanese Patent Application No. Hei11(1999)-239550,proposes an Al—Mg alloy sheet having a texture offering excellent deepdrawability. This Al—Mg alloy sheet has a specified chemicalcomposition, having a texture including grains with a volume fraction of30% to 50% in the Cube orientation and a volume fraction of 10% to 20%in the Brass orientation, in which the grain size is from 50 to 100 μm.

Likewise, Japanese Patent No. 4339869, which is a divisional applicationof Japanese Patent Application No. Hei11(1999)-239550, proposes an Al—Mgalloy sheet having excellent deep drawability. This Al—Mg alloy sheethas a specified chemical composition, includes a texture with a ratio(S/Cube) of the volume fraction in the S orientation to the volumefraction in the Cube orientation of 1 or more and a volume fraction inthe GOSS orientation of 5% or less, in which the grain size is from 20to 100 μm.

SUMMARY OF INVENTION

Some sheets (rolled sheets) are used as forming materials to besubjected particularly to press forming to form the inner panels andother products having complicated shapes with high (deep) side walls.These sheets require low elongation anisotropy (in-plane anisotropy), inaddition to good formability evaluated by common evaluations such asevaluation by Erichsen value. The elongation anisotropy is determined asa difference in elongation (total elongation) between a direction (0°direction) parallel to the sheet rolling direction and a direction (90°direction) perpendicular to the sheet rolling direction.

However, the aluminum alloy sheets according to the techniques disclosedin Japanese Patent No. 4326009 and Japanese Patent No. 4339869, whichare intended to give better deep drawability by the action of thespecific textures, and the conventional 5xxx aluminum alloy sheetsregarding their textures are still susceptible to improvementsparticularly in the elongation anisotropy.

The present invention has been made while focusing on thesecircumstances and has an object to provide a 5xxx aluminum alloy sheetfor forming, which has low elongation anisotropy (in-plane anisotropy)and offers excellent press formability while having strength maintainedat high level.

Solution to Problem

To achieve the object, the present invention provides, according to oneembodiment, an aluminum alloy sheet for forming as follows. Thisaluminum alloy sheet is an Al—Mg alloy sheet containing Mg in a contentof 3.5 to 5.5 mass %, Mn in a content of 0.03 to 0.60 mass %, Cu in acontent of 0.001 to 0.50 mass %, and Zn in a content of 0.001 to 0.50mass %, with the remainder consisting of Al and unavoidable impurities.In a texture of the aluminum alloy sheet in a plane parallel to thesheet surface, the orientation densities of Brass orientation and Sorientation relative to a random orientation of each less than 5, wherethe plane is positioned at a depth half the thickness of the aluminumalloy sheet, and where the orientation densities are measured bySEM-EBSD analysis and expressed by orientation distribution functions(ODFs).

The inventors of the present invention made investigations anew on therelationship between the texture and the elongation anisotropy (in-planeanisotropy) of a 5xxx aluminum alloy sheet, and found that specificcrystal orientations in the texture significantly affect the elongationanisotropy. The specific crystal orientations are two orientations,i.e., the Brass orientation and the S orientation. The inventors furtherfound that the aluminum alloy sheet can have controlled, lowerelongation anisotropy while having strength maintained at high level, bycontrolling and minimizing the orientation densities expressed by ODFsof the two crystal orientations as low as possible, in other words, theelongation anisotropy can be minimized with decreasing orientationdensities expressed by ODFs of the two crystal orientations.

This configuration according to the embodiment of the present inventionallows the aluminum alloy sheet to have significantly better pressformability into, in particular, the inner panels and other productswhich have complicated shapes and have high (deep) side walls.

DESCRIPTION OF EMBODIMENTS

The aluminum alloy sheet according to the embodiment of the presentinvention will be described in detail below on a condition-to-conditionbasis.

Chemical Composition

Initially, the chemical composition of the Al—Mg 5xxx aluminum alloysheet according to the embodiment of the present invention will bedescribed below, on the precondition that the aluminum alloy sheet hasformability and strength at satisfactory levels necessary for use informing into the automobile members such as inner panels, and isproduced through rolling under conditions without significant changes.

The 5xxx aluminum alloy sheet as above contains, in its chemicalcomposition, Mg in a content of 3.5 to 5.5 mass %, Mn in a content of0.03 to 0.60 mass %, Cu in a content of 0.001 to 0,50 mass %, and Zn ina content of 0.001 to 0.50 mass %, with the remainder consisting of Aland unavoidable impurities.

Next, the ranges, significance, and permissible levels of the contentsof the elements in the 5xxx aluminum alloy sheet will be describedbelow. All % ages as the element contents are in mass %.

Ma Content: 3.5 to 5.5 Mass %

Magnesium (Mg) is dissolved (solid-solutionized) in the matrix, therebyallows the aluminum alloy sheet to have better work hardenability and tosurely offer strength and durability necessary as an aluminum alloymaterial sheet for forming, and is essential. The aluminum alloy sheet,when having a Mg content of 3.5 mass % or more, can enjoy the operationand effects at high levels and can resist reduction in pressformability. From the viewpoint of offering the effects at betterlevels, the Mg content is preferably 4.0 mass % or more, and morepreferably 4.5 mass % or more. On the other hand, the aluminum alloysheet, when having a Mg content of 5.5 mass % or less, can prevent sheetproduction issues, such as cracking during hot rolling (reduction indeformability at high temperatures), and prevent reduction in resistanceto use environment, such as stress corrosion cracking resistance. Fromthe viewpoint of offering the effects at better levels, the Mg contentis preferably 5.3 mass % or less, and more preferably 5.0 mass % orless.

Mn Content: 0.03 to 0.60 Mass %

Manganese (Mn) effectively allows the aluminum alloy sheet to havehigher strength by solid-solution strengthening and to have better pressformability by refinement of grain microstructures, and is essential.The aluminum alloy sheet, when having a Mn content of 0.03 mass % ormore, can enjoy the operation and effects at high levels and can meetthe condition for the after-mentioned performance of 0.2% proof stressof 100 MPa or more. From the viewpoint of offering the effects at betterlevels, the Mn content is preferably 0.20 mass % or more, and morepreferably 0.30 mass % or more. On the other hand, the aluminum alloysheet, when having a Mn content of 0.60 mass % or less, may includesmaller amounts of coarse particles and precipitates containing thiselement and less undergoes reduction in press formability. From theviewpoint of offering the effects at better levels, the Mn content ispreferably 0.50 mass % or less, and more preferably 0.40 mass % or less.

Cu Content: 0.001 to 0.50 Mass %

Copper (Cu) effectively allows the aluminum alloy sheet to have higherstrength by solid-solution strengthening and is essential. The aluminumalloy sheet, when having a Cu content of 0.001 mass % or more, tends toenjoy the effect satisfactorily. From the viewpoint of offering theeffect at better level, the Cu content is preferably 0.01 mass % ormore, more preferably 0.10 mass % or more, and furthermore preferably0.20 mass % or more. On the other hand, the aluminum alloy sheet, whenhaving a Cu content of 0.50 mass % or less, is advantageous also in viewof production, namely, does not suffer from reduction in slabcastability, such as cracking in the slab, and less fails to obtainproblem-free slabs to be subjected typically to rolling. From theviewpoint of offering the effects at better levels, the Cu content ispreferably 0.40 mass % or less, and more preferably 0.30 mass % or less.

Zn Content: 0.001 to 0.50 Mass %

Zinc (Zn) effectively allows the aluminum alloy sheet to have higherstrength by solid-solution strengthening and to offer better pressformability, and is essential. The aluminum alloy sheet, when having aZn content of 0.001 mass % or more, can enjoy these effectssufficiently. From the viewpoint of offering the effects at betterlevels, the Zn content is preferably 0.01 mass % or more, and morepreferably 0.10 mass % or more. On the other hand, the aluminum alloysheet, when having a Zn content of 0.50 mass % or less, less undergoes asignificant age hardening, in which the sheet has increasing strengthwith the elapse of time after sheet production, and can less suffer fromreduction in press formability. From the viewpoint of offering theeffects at better levels, the Zn content is preferably 0.40 mass % orless, and more preferably 0.30 mass % or less.

Unavoidable Impurities

The aluminum alloy for use in the embodiment may contain, as unavoidableimpurities, elements other than above, as a result of selection of rawmaterials to be subjected to melting to make ingots. The contents ofunavoidable impurity elements other than the above-described elementsare controlled within ranges as prescribed for 5xxx alloys typically byAluminum Association (AA) Standards or Japanese industrial Standards(JIS). Specifically, non-limiting examples of the unavoidable impurityelements include Fe, Si, Cr, Ti, Zr, V, Ni, Sn, In, Ga, B, and Sc.Controls are performed so that, among these elements, the Fe content is0.7 mass % or less, the Si content is 0.4 mass % or less, the Cr contentis 0.3 mass % or less, and the Ti content is 0.3 mass % or less; and sothat the contents of elements other than Fe, Si, Cr, and Ti are each0.05 mass % or less and are in total of 0.15 mass % or less. Theseelements, when contained in contents within the ranges, do not adverselyaffect advantageous effects of the embodiment, not only when containedas unavoidable impurities, but also when added actively.

Texture

On the precondition that the 5xxx aluminum alloy sheet has a chemicalcomposition within the range, the embodiment allows the aluminum alloysheet to have lower elongation anisotropy, by controlling and minimizingthe orientation densities of the specific crystal orientations in thetexture of the aluminum alloy sheet, where the specific crystalorientations significantly affect the elongation anisotropy, and wherethe orientation densities are expressed by ODFs. The specific crystalorientations are two crystal orientations, i.e., the Brass orientationand the S orientation.

The aluminum alloy sheet can have lower elongation anisotropy whilehaving strength maintained at high level, by minimizing the orientationdensities, as expressed by ODFs, of these two crystal orientations. Inother words, the aluminum alloy sheet can have lower elongationanisotropy with decreasing orientation densities, as expressed by ODFs,of the two crystal orientations. The control to have lower elongationanisotropy as above allows the 5xxx aluminum alloy sheet to havesignificantly better press formability, in particular, into theautomobile panels such as inner panels, which have complicated productshapes and have high (deep) side walls, while the aluminum alloy sheethas strength maintained at high level.

Specifically, the orientation densities of the Brass orientation and theS orientation relative to a random orientation are controlled andminimized each to be less than 5, in a texture of the 5xxx aluminumalloy sheet in a plane parallel to the sheet surface, where the plane ispositioned at a depth half the thickness of the aluminum alloy sheet,and where the orientation densities are determined by SEM-EBSD analysisand are expressed by orientation distribution functions (ODFs).Preferably, in the texture in the aluminum alloy sheet, the orientationdensities of the Brass orientation and the S orientation relative to arandom orientation are further minimized each to be less than 2, wherethe orientation densities are expressed by ODFs. As used herein, theterm “orientation density expressed by ODF relative to a randomorientation” refers to the distribution density of each crystalorientation, while the orientation density, as expressed by ODF, of asample having a random texture is defined as 1. Also as used herein, theterm “random orientation” is defined as a state where all orientationsoccur at an equal probability (completely randomized state).

This configuration allows the aluminum alloy sheet to have, aselongation anisotropy, a difference in elongation (total elongation) of2% or less between a tensile direction (sheetrolling direction) parallelto the sheet rolling direction and a tensile direction perpendicular tothe sheet rolling direction, when JIS No. 5 test specimens are sampledfrom the aluminum alloy sheet and are subjected to tensile tests at roomtemperature. Specifically, the aluminum alloy sheet can have a loweredelongation anisotropy of 2% or less, where the elongation anisotropy ΔElis determined by the equation:

ΔEl=(Total elongation(%)in the sheet rolling direction)−(Totalelongation(%)in a direction perpendicular to the sheet rollingdirection)

As used herein, the term “room temperature” refers to a temperature offrom 20° C. to 25° C.

While the Brass orientation and the S orientation significantly affectthe elongation anisotropy, other orientations such as four orientations,i.e., the Cube orientation, the ND-rotated Cube orientation, the Cuorientation, and the Goss orientation are considered to affect theelongation anisotropy considerably. Thus, the orientation densities ofsuch other orientations relative to a random orientation are preferablyminimized to be less than 5, where the orientation densities areexpressed by ODFs.

However, it may be impossible to reduce the orientation densitiesexpressed by ODFs of the Brass orientation and the S orientation to 0relative to a random orientation, because of production limitations ofsuch 5xxx aluminum alloy sheets. This is also true for the other fourcrystal orientations, i.e., the Cube orientation, ND-rotated Cubeorientation, Cu orientation, and Goss orientation.

In contrast, the aluminum alloy sheet, if having an orientation densityexpressed by ODF of 5 or more for at least one of the Brass orientationand the S orientation relative to a random orientation, makes littledifference in texture from the conventional 5xxx aluminum alloy sheetsand fails to have a low elongation anisotropy of 2% or less. This is thereason for which the conventional 5xxx aluminum alloy sheets fail tohave low elongation anisotropy.

Texture Measurement Method

The crystal orientations specified in the embodiment as above aremeasured by a crystal orientation analysis technique using a fieldemission scanning electron microscope (FE-SEM) equipped with an electronback-scattered diffraction pattern (EBSD) analysis system. A test sampleto be subjected to the measurement is sampled from the sheet at anyposition in a plane parallel to the sheet surface, where the plane ispositioned at a depth half the sheet thickness. Of the test sample, aplane parallel to the sheet surface, where the plane is positioned at adepth half the sheet thickness, is prepared so as to be a measurementplane. On the measurement plane, the orientation densities(dimensionless) expressed by ODFs of the Cube orientation, ND-rotatedCube orientation, Brass orientation, Cu orientation, S orientation, andGoss orientation relative to a random orientation are measured by theSEM-EBSD analysis.

The SEM-EBSD analysis is widely used as a technique for measuringcrystal orientations and textures. In the analysis, the sample of thealuminum alloy sheet is placed in the lens barrel of the FE-SEM, andelectron beams are applied to the sample with scanning the samplesurface at 1-μm intervals, while diffraction patterns of the resultingback-scattered electrons are captured into an EBSD analysis system, andcrystal orientations are analyzed, where the EBSD analysis system isexemplified typically by EBSD Measurement-Analysis System OIM(Orientation Imaging Microscopy) Data & Analysis, supplied by TSL. Thisgives an electron back-scattered diffraction pattern (EBSD pattern) ateach point, and the obtained EBSD patterns are indexed to determinecrystal orientations in the electron beam-applied region. Theorientation distribution functions (ODFs) and area % ages are determinedby calculation, on the basis of the resulting crystal orientationmeasurement data resulting from crystal orientation measurement ofallover the measurement region by EBSD analysis. In addition, theanalysis can also give the average grain size of the microstructure(crystal structure). The details of the crystal orientation analysistechnique using an FE-SEM equipped with an EBSD analysis system can befound, in detail, typically in Research and Development, Kobe SteelEngineering Reports Vol. 52, No. 2 (September 2002), pp. 66-70.

Average Grain Size

In the embodiment, the average grain size of the 5xxx aluminum alloysheet is not limited. However, the average grain size is preferablysmaller, and more preferably, specifically, 100 μm or less, so as toallow the aluminum alloy sheet to have better ductility and lowerelongation anisotropy. The lower limit of the average grain size isabout 10 μm in consideration of sheet production limitations.

Strength (0.2% Proof Stress): 100 MPa or More

The aluminum alloy sheet according to the embodiment preferably has a0.2% proof stress of 100 MPa or more. The aluminum alloy sheet, whenhaving a 0.2% proof stress of 100 MPa or more, can surely have strengthnecessary as an alloy sheet for use in automobile members. Accordingly,the aluminum alloy sheet has a 0.2% proof stress of preferably 100 MPaor more, and particularly preferably 120 MPa or more.

The 0.2% proof stress can be controlled by the alloy element contents,and by the thermal hysteresis and rolling reduction in individual steps,among conditions for the steps of the production method mentioned below.

Aluminum Alloy Sheet Thickness

The thickness of the aluminum alloy sheet according to the embodiment isnot limited, but is typically 0.5 mm or more to offer strength andrigidity necessary for automobile member use. On the other hand, thethickness is typically 6.0 mm or less in consideration of limitations informing such as press forming, and in consideration of such an allowablerange of weight increase as not adversely affects the effective weightreduction as compared with comparative (conventionally-used) steelmaterials.

Production Method

Next, a method for producing the aluminum alloy sheet according to theembodiment will be described below.

Melting and Casting

An aluminum alloy having a chemical composition within the range ismelted to give a molten metal, from which an ingot having apredetermined shape is prepared. The way to melt and cast the aluminumalloy is not limited and may employ a common or known procedure.

Homogenization

Next, the aluminum alloy slab is subjected to homogenization (soaking)prior to hot rolling. The homogenization is performed so as tounifoimize or homogenize a heterogenous microstructure formed uponcasting. The homogenization of the slab is performed at a soakingtemperature (attaining temperature) of from 400° C. to lower than thealuminum alloy melting point. The homogenization, if performed at asoaking temperature lower than 400° C., may offer smaller effects;whereas heating up to a temperature equal to or higher than the meltingpoint is unnecessary.

Hot Rolling

Subsequent to the homogenization, the work is subjected to hot rollinginto a sheet having a predetermined thickness. The hot rolling ispreferably started at a temperature of 350° C. or higher, because thehot rolling, if started at an excessively low temperature, may causehigh resistance to deformation during rolling. The hot rolling ispreferably performed so that the sheet during hot rolling passes throughthe temperature range (temperature region) of from 445° C. down to 400°C. within a short time of preferably 20 minutes or shorter, and morepreferably 15 minutes or shorter. This is preferred so as to control thetexture of the sheet to be in the range specified in the embodiment. Thehot rolling, if performed so that the sheet passes through thetemperature range (temperature region) of from 445° C. down to 400° C.over a long time of longer than 20 minutes, may cause at least one ofthe Brass orientation and S orientation to fail to be present in anorientation density, as expressed by ODF, of less than 5 relative to arandom orientation, and may cause the aluminum alloy sheet to havehigher elongation anisotropy. For offering preferred orientationdensities expressed by ODFs, the time for the sheet to pass through thetemperature range (temperature region) is preferably 2 minutes orlonger.

Cold Rolling

The work after the hot rolling may be dealt as a product sheet having apredetermined thickness without being subjected to cold rolling.However, the work is preferably subjected to cold rolling into a productsheet having a predetermined thickness, so as to surely have surfacequality, such as surface roughness, and flatness at satisfactory levels.

Final Annealing Process

The rolled sheet after the hot rolling or after the cold rolling issubjected to final annealing process. The term “final annealing” refersnot to annealing performed selectively according to necessity beforecold rolling or during a pass (or between passes) in the cold rolling,but to annealing finally performed on the product sheet. The finalannealing may be performed either as batch annealing in a batch furnace,or continuous annealing in a continuous annealing furnace. In thecontinuous annealing, a heat treatment is performed at a hightemperature for a short time. However, when the final annealing isperformed as batch annealing, the sheet is preferably held at a sheetattaining temperature of from 300° C. to 450° C. for a time of from 0.5hour to 24 hours. The final annealing, if performed at an annealingtemperature higher than 450° C., tends to cause grains to coarsen, andtends to cause the final product sheet to have an average grain sizegreater than 100 μm and to have lower ductility. The final annealing, ifperformed at an attaining temperature lower than 300° C., tends toimpede recrystallization from proceeding, tends to cause a deformedmicrostructure to remain, and may cause the aluminum alloy sheet to havesignificantly lower elongation, namely, significantly lower ductilityand press formability.

When the final annealing is performed as batch annealing, in which theheating rate is relatively low, the annealing is preferably controlledto be performed by heating up to the annealing temperature at a rate oftemperature rise of 10° C./h (hour) or more, and more preferably 30°C./h (hour) or more. This control is not always necessary when the finalannealing is performed as the continuous annealing, in which the rate oftemperature rise is relatively high. The annealing, if performed at arate of temperature rise of less than 10° C./h, may cause crystalorientations to change, may cause at least one of the Brass orientationand the S orientation to grow excessively and to fail to be present inan orientation density, as expressed by ODF, of less than 5 relative toa random orientation, and may cause the aluminum alloy sheet to havehigher elongation anisotropy.

The final annealing, when performed as continuous annealing, ispreferably performed though heating up to a sheet attaining temperatureof from 400° C. to lower than the aluminum alloy melting point for ashort time of from about 0.1 second to about 60 seconds.

The sheet after the final annealing as above is used as a product sheet,which is a forming material for final use such as the automobile panelmember. Specifically, the sheet is press-formed to have the shape of themember, then generally subjected to a treatment such as paint baking,and is finally used as the member.

EXAMPLES

Next, the aluminum alloy sheet according to the embodiment will beillustrated in further detail with reference to several examples below.Aluminum alloy sheets for forming, having chemical compositions given inTables 1A, 1B and 2, were produced and, after final annealing, wereinvestigated for texture, elongation anisotropy, strength (0.2% proofstress), and formability. Results of these are also given in Tables 1A,1B and 2. So as to have different textures, the samples according to theexamples and the comparative examples in Table 2 were produced underdifferent production conditions, by appropriately adjusting the time fora sample sheet to pass through the temperature range of from 445° C.down to 400° C. in hot rolling, and the rate of temperature rise infinal annealing according to batch system.

Specifically, common production conditions for the sample aluminum alloysheets will be described below. Aluminum alloy slabs having a thicknessof 500 mm and having the chemical compositions given in Tables 1A, 1Band 2 were made by direct chill casting. The slabs were subjected tosurface scalping, then to homogenization at 500° C. for 4 hours, and tohot rolling started at that temperature (500° C.), where the hot rollingwas finished at 300° C. or lower. The resulting hot-rolled sheets eachhad a final thickness of 3.3 mm. In the samples in Tables 1A and 1B, thetime (passing time) for the sheet to pass through the temperature rangeof from 445° C. down to 400° C. in the hot rolling was set to 15 minutesin common with each sample. In contrast, the samples in Table 2 wereproduced at different passing times.

The hot-rolled sheets were subjected to cold rolling to a thickness of1.0 mm in common and process annealing. In the samples in Tables 1A and1B, the cold rolling was performed to a cold rolling reduction of 20% or70%, and in the samples in Table 2, the cold rolling was performed to acold rolling reduction of 20% in common. The cold-rolled sheets weresubjected to final annealing, where the final annealing was performedaccording to batch system or continuous system at the rate oftemperature rise (° C./h) and at the annealing temperature (° C.) forthe holding time (in hour or second) given in Tables 1A, 1B and 2. Abatch annealing furnace was employed in Examples 1 to 20, Examples 28 to35, Comparative Examples 1 to 7, and Comparative Examples 11 to 14; anda continuous annealing furnace was employed in Examples 21 to 27.

Test samples (sheets) were sampled from the aluminum alloy sheets afterthe final annealing and were each investigated and evaluated formechanical properties, texture, and formability according to measurementprocedures as follows.

Mechanical Properties

From the test samples after the final annealing, tensile test specimensto be subjected to mechanical properties measurements were sampled andprepared as JIS No. 5 test specimens (having a width in the parallelportion of 25 mm, a gauge length (GL) of 50 mm, and a thickness of 1 mm)so that the tensile directions be a direction parallel to, and adirection perpendicular to, the sheet rolling direction. The tensiletest specimens were subjected to tensile tests at room temperature (20°C.) and a tensile speed of 5 mm/min. In the tensile tests, 0.2% proofstress and total elongation were measured. Measurements in the tensiletests were perfoimed two times per test sample (N=2), the twomeasurements were averaged, and the average was defined as a value foreach of the properties. The elongation anisotropy was determined in thetensile tests as a difference (%) in elongation between a tensiledirection parallel to the sheet rolling direction and a tensiledirection perpendicular to the sheet rolling direction, namely, wasdetermined according to the equation:

Elongation anisotropy ΔEl=(Total elongation(%)in the sheet rollingdirection)−(Total elongation(%)in a direction perpendicular to the sheetrolling direction)

The 0.2% proof stress was evaluated so that a sample having a 0.2% proofstress of 100 MPa or more was evaluated as being acceptable, whichstrength level is necessary as the automobile inner panel member. Theelongation anisotropy was evaluated by calculating the difference inelongation between the parallel direction and the perpendiculardirection (to the sheet rolling direction), and making an evaluationaccording to criteria as follows: A sample having a difference inelongation of 2% or less was evaluated as being very good and indicatedby “VG”; a sample having a difference in elongation from greater than 2%to 4% was evaluated as being good and indicated by “Good”; and a samplehaving a difference in elongation greater than 4% was evaluated as beingrejected and indicated by “Poor”.

Texture

The texture was determined in the following procedure. Specimens weresampled from the test samples after the final annealing so that anobservation plane was defined as a plane parallel to the sheet surface,where the plane was located at a position of a depth half the sheetthickness in the through-thickness direction The texture of theobservation plane of each sample was measured on the observation planein a region of 600 μm in the sheet rolling direction by 600 μm in thesheet transverse direction at step intervals of 1 μm, using an FE-SEMJSM-7000F (supplied by JEOL Ltd.) at an acceleration voltage of 20 kVand using an EBSD measurement software TSL-OIM (Orientation ImagingMicroscopy)-Data Collection Ver. 5 (supplied by TSL Solutions KK). Asused herein the term “through-thickness direction” refers to a directionparallel to the thickness of the sheet to be measured.

The resulting measurement data were analyzed using an analysis softwareTSL-OIM (Orientation Imaging Microscopy) Analysis Ver. 7 (supplied byTSL Solutions KK), which is an EBSD analysis software, and theorientation densities expressed by ODFs of the Cube orientation,ND-rotated Cube orientation, Brass orientation, Cu orientation, Sorientation, and Goss orientation relative to a random orientation weredeteimined in a plane of the sheet, where the plane was parallel to thesheet surface and positioned at a depth half the thickness in thethrough-thickness direction.

The specimens to be subjected to the measurement were sampled in anumber of 5 per test sample (N=5) at any positions in a plane of thesheet, where the plane was parallel to the sheet surface and positionedat a depth half the sheet thickness. The samples were subjected tomechanical polishing and buffing, and further to ion sputtering using anelectron spectroscopy for chemical analysis (ESCA) system, to give anobservation plane, as a measurement plane, where the observation planewas parallel to the sheet surface and positioned at a depth half thesheet thickness. The measurement results of the five test specimensmeasured by the SEM-EBSD analysis were averaged, and the averages weredefined as the orientation densities (dimensionless) expressed by ODFsspecified in the embodiment.

On the basis of these measurement results, a sample having orientationdensities expressed by ODFs of the Brass orientation and the Sorientation of each less than 5 relative to a random orientation wasevaluated as having low elongation anisotropy and as being good. Asample having orientation densities expressed by ODFs of the Brassorientation and the S orientation of each less than 2 relative to arandom orientation was evaluated as having still lower elongationanisotropy and as being very good (VG). In contrast, a sample having anorientation density as expressed by ODF of at least one of the Brassorientation and the S orientation of more than 5 relative to a randomorientation was evaluated as having high elongation anisotropy and asbeing poor.

In this connection, all the examples and the comparative examples hadaverage grain sizes of from 10 to 30 μm in the sheet rolling direction,where the average grain sizes were measured by the SEM-EBSD analysis onthe plane being parallel to the sheet surface and positioned at a depthhalf the sheet thickness.

Erichsen Cupping Test

The test samples after the final annealing were subjected to Erichsencupping tests so as to evaluate formability. The formability wasevaluated in accordance with Method B of Erichsen cupping testprescribed in JIS Z 2247:1977. A sample having an Erichsen value of 9.0mm or more was evaluated as having good formability (as beingacceptable); and a sample having an Erichsen value less than 9.0 mm wasevaluated as having poor formability (as being rejected).

The examples in Tables 1A and 1B had chemical compositions within therange specified for the aluminum alloy sheet according to theembodiment, and were produced under production conditions for the hotrolling and final annealing within preferred ranges. The examples inTables 1A and 1B therefore had textures as specified in the embodiment,had strength maintained at a level required of 5xxx aluminum alloysheets, still had low elongation anisotropy between a direction parallelto, and a direction perpendicular to, the sheet rolling direction, andoffered good formability as evaluated by the Erichsen value.

In contrast, Comparative Examples 1 to 10 in Tables 1A and 1B hadchemical compositions out of the range specified in the presentinvention, although these samples were produced under conditions for hotrolling and final annealing within the preferred ranges. The samplestherefore had an excessively low strength or offered poor formabilityevaluated by the Erichsen value, because of having chemical compositionsout of the range specified in the embodiment, although these samples hadrelatively low elongation anisotropy.

Comparative Example 8 had a Mg content out of the chemical compositionrange specified in the embodiment, thereby occurred cracking in hotrolling, and failed to be produced through subsequent steps, by whichthe textures and the properties could not be evaluated. ComparativeExamples 9 and 10 had a content of at least one of Cu and Zn out of thechemical composition range specified in the embodiment, thereby occurredcracking in slab casting, and failed to be produced through subsequentsteps, by which the textures and the properties could not be evaluated.The symbol “-” in Tables 1A and 1B means that the correspondingproduction condition was not applied (corresponding production step wasnot performed).

The examples in Table 2 had aluminum alloy sheet chemical compositionswithin the range specified in the embodiment, and were produced underproduction conditions within preferred ranges, for the time for thealuminum alloy sheet to pass through the temperature range of from 445°C. down to 400° C. in the hot rolling step and for the rate oftemperature rise in the final annealing. The examples in Table 2therefore also had textures as specified in the embodiment, had strengthmaintained at a level required of 5xxx aluminum alloy sheets, still hadlow elongation anisotropy between a direction parallel to, and adirection perpendicular to, the sheet rolling direction, and offeredgood formability as evaluated by the Erichsen value.

In contrast, the comparative examples in Table 2 had aluminum alloysheet chemical compositions within the range specified in theembodiment, but were produced under a production condition(s) out of thepreferred range(s), for at least one of the time for the aluminum alloysheet to pass through the temperature range from 445° C. down to 400° C.in the hot rolling step, and the rate of temperature rise in the finalannealing. These comparative examples therefore tended to have texturesout of the range specified in the embodiment and, in particular, had anorientation density expressed by ODF of at least one of the Brassorientation and the S orientation of 5 or more relative to a randomorientation. As a result, the comparative examples had higher elongationanisotropy, lower Erichsen indices, and offered lower press formability,as compared with the examples.

Comparative Examples 11 to 13 in Table 2 underwent a hot rolling step inwhich the aluminum alloy sheet passed through the temperature range from445° C. down to 400° C. over an excessively long time of longer than 20minutes. Comparative Example 14 in Table 2 underwent final annealingperformed at an excessively low rate of temperature rise of less than10° C./h.

Samples, when having a texture out of the range specified in theembodiment and having high elongation anisotropy as with the comparativeexamples in Table 2, have lower Erichsen values as with the comparativeexamples in Table 2, because the values are affected by deformability ina direction with lower elongation. However, the Erichsen value itself issignificantly affected not only by elongation anisotropy, but also byother metallurgical factors than the texture, such as alloy chemicalcomposition, grain size, and precipitate state, and acts as an index ofoverall formability. Accordingly, a high Erichsen value does not alwaysmean low elongation anisotropy. Specifically, a known sample having ahigh Erichsen value is not always considered to have low elongationanisotropy.

These results support the significance of the chemical composition andthe texture specified in the embodiment and the significance ofpreferred production conditions to give these properties, so as toprovide lower elongation anisotropy and better press formability whilehaving strength maintained at high level.

TABLE 1A Passing time Final- Aluminum alloy (min) annealed sheetchemical through Cold sheet composition (mass temperature rolling Final(annealed %, the remainder being range from reduction annealingconditions material) Al and unavoidable 445° C. down (%) before Rate ofstrength impurities) to 400° C. in final temperature Holding 0.2% ProofCategory Mg Mn Cu Zn hot rolling annealing rise condition stress (MPa)Ex. 1  3.7 0.25 0.02 0.01 15 20 50° C./h at 104 Ex. 2  4.2 0.25 0.020.01 380° C. 108 Ex. 3  4.7 0.25 0.02 0.01 for 4 h 116 Ex. 4  5.3 0.250.02 0.01 125 Ex. 5  4.7 0.05 0.02 0.01 107 Ex. 6  4.7 0.35 0.02 0.01123 Ex. 7  4.7 0.55 0.02 0.01 128 Ex. 8  4.7 0.25 0.40 0.01 128 Ex. 9 4.7 0.25 0.02 0.30 121 Ex. 10 4.7 0.25 0.02 0.45 124 Ex. 11 3.7 0.400.05 0.05 103 Ex. 12 4.2 0.10 0.15 0.15 109 Ex. 13 4.7 0.50 0.20 0.15124 Ex. 14 4.5 0.30 0.30 0.30 118 Ex. 15 3.7 0.25 0.02 0.01 70 116 Ex.16 4.2 0.25 0.02 0.01 129 Ex. 17 4.7 0.25 0.02 0.01 138 Ex. 18 5.3 0.250.02 0.01 146 Ex. 19 4.7 0.12 0.02 0.01 133 Ex. 20 4.7 0.25 0.02 0.30143 Texture (orientation density expressed by ODF) Form- ND- abilityrotated (Erich- Cube Cube Brass Cu S Goss Elonga- sen orien- orien-orien- orien- orien- orien- tion value) Category tation tation tationtation tation tation anisotropy (mm) Ex. 1  1 1 3 2 3 1 Good 9.2 Ex. 2 1 1 3 2 3 1 Good 9.5 Ex. 3  1 1 3 2 3 1 Good 9.5 Ex. 4  1 1 3 2 3 1 Good9.6 Ex. 5  1 1 3 2 3 1 Good 9.5 Ex. 6  1 1 3 2 3 1 Good 9.6 Ex. 7  1 1 32 3 1 Good 9.7 Ex. 8  1 1 4 3 2 0 Good 9.4 Ex. 9  1 0 3 2 3 1 Good 9.8Ex. 10 1 0 3 2 3 1 Good 9.9 Ex. 11 1 0 3 2 3 1 Good 9.5 Ex. 12 1 0 4 3 20 Good 9.6 Ex. 13 1 0 4 3 2 0 Good 9.6 Ex. 14 1 0 4 3 2 0 Good 9.7 Ex.15 1 1 3 4 3 2 Good 9.1 Ex. 16 1 1 4 2 3 2 Good 9.4 Ex. 17 1 1 4 2 3 2Good 9.4 Ex. 18 1 1 3 4 3 2 Good 9.5 Ex. 19 1 1 3 2 3 2 Good 9.3 Ex. 201 0 4 3 3 1 Good 9.6

TABLE 1B Passing time Final- Aluminum alloy (min) annealed sheetchemical through Cold sheet composition (mass %, temperature rollingFinal (annealed the remainder being range from reduction annealingconditions material) Al and unavoidable 445° C. down (%) before Rate ofstrength impurities) to 400° C. in final temperature Holding 0.2% ProofCategory Mg Mn Cu Zn hot rolling annealing rise condition stress (MPa)Ex. 21 3.7 0.25 0.02 0.01 15 70 5000° C./h at 102 Ex. 22 4.2 0.25 0.020.01 500° C. 115 Ex. 23 4.7 0.25 0.02 0.01 for 124 Ex. 24 5.3 0.25 0.020.01 30 sec 130 Ex. 25 4.7 0.12 0.02 0.01 119 Ex. 26 4.7 0.25 0.10 0.01127 Ex. 27 4.7 0.25 0.02 0.30 128 Comp. 3.3 0.25 0.02 0.01 20  50° C./hat 93 Ex. 1 380° C. Comp. 4.7 0.01 0.02 0.01 for 4 h 102 Ex. 2 Comp. 4.70.70 0.02 0.01 135 Ex. 3 Comp. 3.6 0.25 0.02 0.0005 98 Ex. 4 Comp. 4.70.25 0.02 0.60 151 Ex. 5 Comp. 3.6 0.25 0.0005 0.01 97 Ex. 6 Comp. 3.30.60 0.40 0.40 117 Ex. 7 Comp. 5.7 0.25 0.02 0.01 — — — — Cracked in hotrolling Ex. 8 and could not be produced through subsequent steps Comp.4.7 0.25 0.60 0.01 — — — — Cracked in slab casting Ex. 9 and could notbe produced through subsequent steps Comp. 4.7 0.30 0.60 0.60 — — — —Cracked in slab casting Ex. 10 and could not be produced throughsubsequent steps Texture (orientation density expressed by ODF) ND-Form- rotated ability Cube Cube Brass Cu S Goss Elonga- (Erichsen orien-orien- orien- orien- orien- orien- tion value) Category tation tationtation tation tation tation anisotropy (mm) Ex. 21 2 3 0 0 0 1 VG 9.2Ex. 22 2 3 0 0 0 1 VG 9.5 Ex. 23 2 3 0 0 0 1 VG 9.6 Ex. 24 2 3 0 0 0 1VG 9.7 Ex. 25 2 3 0 0 0 1 VG 9.4 Ex. 26 2 3 0 0 0 1 VG 9.6 Ex. 27 2 3 00 0 1 VG 9.7 Comp. 1 1 3 2 3 1 Good 9.0 Ex. 1 Comp. 1 1 3 2 3 1 Good 8.8Ex. 2 Comp. 1 1 3 3 3 1 Good 8.9 Ex. 3 Comp. 1 1 3 2 3 1 Good 9.3 Ex. 4Comp. 1 0 3 2 3 1 Good 8.9 Ex. 5 Comp. 1 1 3 2 3 1 Good 9.3 Ex. 6 Comp.1 0 3 2 3 1 Good 8.7 Ex. 7 Comp. Cracked in hot rolling and could not beproduced through subsequent steps Ex. 8 Comp. Cracked in slab castingand could not be produced through subsequent steps Ex. 9 Comp. Crackedin slab casting and could not be produced through subsequent steps Ex.10

TABLE 2 Passing time Final- (min) annealed Aluminum alloy sheet throughCold sheet chemical composition temperature rolling Final (annealed(mass %, the remainder being range from reduction annealing conditionsmaterial) Al and unavoidable 445° C. down (%) before Rate of strengthimpurities) to 400° C. in final temperature Holding 0.2% Proof CategoryMg Mn Cu Zn hot rolling annealing rise condition stress (MPa) Ex. 28 4.70.25 0.02 0.01 11 20 50° C./h at 118 Ex. 29 4.7 0.25 0.02 0.01 15 380°C. 116 Ex. 30 4.7 0.25 0.02 0.01 18 for 4 h 115 Ex. 31 3.7 0.25 0.020.01 18 102 Ex. 32 4.2 0.25 0.02 0.01 18 107 Ex. 33 3.7 0.40 0.05 0.0515 103 Ex. 34 4.5 0.30 0.30 0.30 12 120 Ex. 35 4.7 0.25 0.02 0.01 15 15°C./h 116 Comp. 4.7 0.25 0.02 0.01 23 50° C./h 116 Ex. 11 Comp. 3.7 0.400.05 0.05 22 100 Ex. 12 Comp. 4.5 0.30 0.30 0.30 25 119 Ex. 13 Comp. 4.70.25 0.02 0.01 15 115 Ex. 14  6° C./h Texture (orientation densityexpressed by ODF) ND-rotated Formability Cube Cube Brass Cu S GossElongation (Erichsen value) Category orientation orientation orientationorientation orientation orientation anisotropy (mm) Ex. 28 1 1 3 2 2 1Good 9.4 Ex. 29 1 1 3 2 3 1 Good 9.5 Ex. 30 1 1 3 2 3 1 Good 9.5 Ex. 311 1 3 2 3 1 Good 9.2 Ex. 32 1 1 3 2 3 1 Good 9.5 Ex. 33 1 0 3 2 3 1 Good9.5 Ex. 34 1 0 3 3 2 1 Good 9.7 Ex. 35 1 1 4 2 3 0 Good 9.4 Comp. 1 1 63 5 2 Poor 8.7 Ex. 11 Comp. 1 0 6 3 4 2 Poor 8.8 Ex. 12 Comp. 1 0 6 3 52 Poor 8.7 Ex. 13 Comp. 1 1 6 2 5 2 Poor 8.8 Ex. 14

INDUSTRIAL APPLICABILITY

The embodiment of the present invention can provide 5xxx aluminum alloysheets for forming, which aluminum alloy sheets have low elongationanisotropy (in-plane anisotropy) and excellent press formability whilehaving strength maintained at high level. This can expand theapplications of 5xxx aluminum alloy sheets to forming typically into theautomobile members.

1. An aluminum alloy sheet comprising: Mg in a content of 3.5 to 5.5mass %; Mn in a content of 0.03 to 0.60 mass %; Cu in a content of 0.001to 0.50 mass %; Zn in a content of 0.001 to 0.50 mass %, Al andunavoidable impurities, wherein orientation densities of Brassorientation and S orientation relative to a random orientation are eachless than 5 in a texture of the aluminum alloy sheet, in a planeparallel to a sheet surface, where the plane is positioned at a depthhalf the thickness of the aluminum alloy sheet, and where theorientation densities are measured by SEM-EB SD analysis and areexpressed by orientation distribution functions (ODFs).
 2. The aluminumalloy sheet according to claim 1, wherein the orientation densities ofthe Brass orientation and the S orientation are each less than 2 in thetexture of the aluminum alloy sheet.
 3. The aluminum alloy sheetaccording to claim 1, wherein the aluminum alloy sheet has suchelongation anisotropy that a JIS No. 5 test specimen sampled from thealuminum alloy sheet, when subjected to a tensile test at roomtemperature, has a difference in elongation of 2% or less between atensile direction parallel to a sheet rolling direction and a tensiledirection perpendicular to the sheet rolling direction.
 4. An automobilemember formed from the aluminum alloy sheet according to claim
 1. 5. Thealuminum alloy sheet according to claim 2, wherein the aluminum alloysheet has such elongation anisotropy that a JIS No. 5 test specimensampled from the aluminum alloy sheet, when subjected to a tensile testat room temperature, has a difference in elongation of 2% or lessbetween a tensile direction parallel to a sheet rolling direction and atensile direction perpendicular to the sheet rolling direction.